One of the most significant developments in semiconductor technology in recent years has been the increased use and importance of compound semiconductors. Particularly significant are the Group III-V semiconductor compound devices composed of elements of Groups III and V of the periodic table, such as gallium arsenide (GaAs) and indium phosphide (InP). Compound semiconductors are used in such devices as lasers, light-emitting diodes, microwave oscillators and amplifiers, and various types of light detectors.
Most such commercial use of compound semiconductors requires the growth of large single-crystal ingots from which wafers can be cut for the subsequent fabrication of useful devices. One of the more promising methods for such crystal growth is the vertical gradient freeze (VGF) method, particularly the VGF method defined in the W. A. Gault U.S. Pat. No. 4,404,172, granted Sept. 13, 1983, and assigned to Western Electric Company, Inc., which is hereby incorporated herein by reference. According to this method, polycrystalline starting material is placed in a vertically extending crucible including a small cylindrical seed well portion at its bottom end which snugly contains a seed crystal. Initially, the starting material and a portion of the seed crystal are melted. The power to the system is then reduced in such a manner that freezing proceeds vertically upwardly from the seed crystal. The major advantage of the VGF method is that crystals with very low dislocation densities can be produced using low thermal gradients and slow rates of cooling.
It is well-known that the Group III-V compounds tend to dissociate at higher temperatures, with the more volatile Group V element escaping into the vapor phase. Several approaches have been developed to prevent or retard this tendency during crystal growth. For example, in one approach to the growth of InP, escape of the more volatile phosphorus component is retarded by providing a vapor pressure of phosphorus vapor over the melt from a separately heated reservoir of phosphorus within the sealed growth container. It is also known that Group V material loss from the melt may be retarded with the use of any of various materials such as boric oxide (B.sub.2 O.sub.3), barium chloride (BaCl.sub.2), or calcium chloride (CaCl.sub.2) which act as diffusion barriers. Such additives, having a lower density than the molten indium phosphide, rise to the surface, encapsulate the melt, and, together with an inert gas pressure in the vessel, can contain the volatile vapors; see, for example, the paper of "Growth of Single Crystals of GaAs in Bulk and Thin Film Form," by B. A. Joyce, included in the book, Crystal Growth, edited by B. R. Pamplin, Pergamon Press, 1975, pp. 157-184 at p. 165 and the paper by J. B. Mullen et al., "Liquid Encapsulation Techniques: The Use of an Inert Liquid in Suppressing Dissociation During the Melt-Growth of InAs and GaAs Crystals," Journal of Physical Chem. Solids, Volume 26, pp. 782-784, 1965.
One problem with using a liquid encapsulant as described above is that, after the crystal freezing process, the liquid encapsulant may flow into a gap between the frozen single crystal and the crucible wall. Subsequently, the encapsulant in the gap itself freezes, which creates stresses that can damage either the crystal or the crucible.